Process for purifying a catholyte used for electrolytic hydrodimerization of acrylonitrile

ABSTRACT

In a process for producing adiponitrile by electrolyzing a mixture containing acrylonitrile, a supporting electrolyte and water, the improvement characterized in that said electrolysis is carried out by removing acrylonitrile from a catholyte contaminated with metal ions, reacting the metal ions contained in the catholyte with carbon dioxide in an alkaline region of a pH value of not lower than 10 thereby precipitating the contaminant metal ions as carbonates thereof, removing thus precipitated carbonates therefrom and recycling the resulting purified solution of the supporting electrolyte as a catholyte.

United States Patent Maomi Seko Tokyo;

Akira Yomiyama, Nobeokashi, Miyazakiken; Shinsaku Ogawa, Nobeoka-shi,Miyazaki-ken; Ryozo Komori, Nobeokashi, Miyazaki-ken; Muneo Yoshida,Nobeoka-shi, Miyazaki-ken, all of Japan [21] Appl. No. 835,193

[72] Inventors [22] Filed June 20, 1969 [45] Patented Oct. 26, 1971 [73]Assignee Asahi Kasei Kogyo Kabushiki Kaisha Osaka, Japan [32] PriorityJune 26, 1968 Japan [54] PROCESS FOR PURIFYING A CATHOLYTE USED FORELECTROLYTIC HYDRODIMERIZATION OF ACRYLONITRILE 7 Claims, 1 Drawing Fig.

[52] U.S. Cl 20fi/73A 51 in. CI c071 29/06 50 Field of Search 204/7244[56] References Cited FOREIGN PATENTS 1,548,304 12/1968 France PrimaryExaminer-F. C. Edmundson Attorney-Flynn & Frishauf PROCESS FOR PURIFYINGA CATHOLYTE USED FOR ELECTROLYTIC HYDRODIMERIZATION OF ACRYLONITRILEBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to a process for purifying a catholyte containing metal ions ascontaminants obtained in the electrolytic hydrodimerization ofacrylonitrile for producing adiponitrile.

2. Description of the Prior Art There have been known heretofore variousprocesses for obtaining adiponitrile by electrolytically hydrodimerizingacrylonitrile, for example, as disclosed in U.S. Pat. No. 3,193,481 andothers in which an aqueous solution containing acrylonitrile andadiponitrile in high concentrations is electrolyzed using quaternaryammonium salts having oleophilic anions as a supporting electrolyte.There have also been known processes as disclosed in Dutch Pat.Specification No. 6,708,254 and others in which acrylonitrile andadiponitrile in a suspension state are electrolyzed using quaternaryammonium salts having nonoleophilic anions as a supporting electrolyte.

It has been proposed heretofore as disclosed in U.S. Pat. No. 3,193,480that adiponitrile be produced by the electrolysis of acrylonitrile in aelectrolytic cell divided by an ion exchange membrane to form an anodecompartment and a cathode compartment using solutions as described aboveas a catholyte.

However, when the electrolytic hydrodimerization processes as proposedheretofore are practiced on a commercial scale for a prolonged period oftime, there are intermixed into a catholyte a variety of metals derivedfrom materials constituting an anode, electrolytic cell, pipes, pumpsand appendant equipment as well as from an anolyte and water beingemployed, or ever from equipment used for the preparation of supportingelectrolytes.

These metals primarily include, for example, lead, iron, calcium,magnesium, silver, nickel, chromium and zinc. When these metals areaccumulated in a catholyte, they tend to be converted into hydroxidesthereof by a layer of concentrated alkali being present on a cathodesurface to precipitate thereon, or, in other occasion, electrolyticallyreduced to a metallic condition with consequential precipitation ofmetals on the cathode surface.

When the precipitation of these metals on the cathode surface occurs,however small in amounts, e.g., in the vicinity of 50 mgJdm, the supplyof acrylonitrile to the surface of the electrode is hindered, leading toinconveniences in byproduct formation of propionitrile and in generationof hydrogen gas. This is quite undesirable because, by hinderancesdescribed above, the yield of adiponitrile is lowered and so is theelectric current efficiency.

In order to prevent such phenomenon as mentioned above from occurring,there has been known a process for electrolytically purifying acatholyte as disclosed in the specification of Dutch Pat. ApplicationNo. 6,515,510, or a process described in that of Dutch Pat. ApplicationNo. 6,607,654 in which a catholyte is heattreated after being made toalkaline.

Particularly, in the process of Dutch Pat. Application No. 6,607,654referred to above, when producing adiponitrile by electrolytichydrodimerization of acrylonitrile using a catholyte of an aqueoussolution of a hydrotropic quaternary ammonium salt having a pH rangingfrom 6 to l2l, impurities are removed from the catholyte by acombination of steps comprising:

a. firstly, separating the solution of the hydrotropic quaternaryammonium salt from the catholyte,

b. secondly, adjusting thus separated solution of ammonium salt to havea pH of at least 7 or higher and a concentration of the ammonium salt inthe range of from 28 to 85 percent, heating thus adjusted solution at atemperature not higher than 115 C., and

c. lastly, separating impurities from the solution of ammonium salt.

The prior an process known heretofore such as referred to above isaccompanied by a drawback in that since a catholyte is heatednecessarily under a strong alkaline condition, there occur undesirableside reactions such as, e.g., thermal decomposition of quaternaryammonium salts due to Hoffmann decomposition, hydrolysis ofacrylonitrile, adiponitrile or other byproducts, and polymerization ofacrylonitrile.

SUMMARY OF THE INVENTION It is, accordingly, an object of the presentinvention to provide a process for purifying a contaminated catholyteobtained in the electrolytic hydrodimerization of acrylonitrile, freefrom the drawbacks of the prior art processes mentioned above.

It has been found that the object of the present invention mentionedabove can be preferably accomplished by removing metal ions accumulatedin a catholyte due to the electrolytic hydrodimerization ofacrylonitrile from a catholyte in the form of carbonates thereof. Thus,the present invention has its basis on this novel finding.

More particularly, the present invention relates to a process forpurifying a catholyte used for electrolytically hydrodimerizingacrylonitrile in an electrolytic cell divided by a diaphragm to form ananode compartment and a cathode compartment, which comprises blowing agaseous carbon dioxide into the catholyte containing metal ions therebymaking the catholyte alkaline, particularly at a pH of not lower than10, precipitating the metal ions in the form of carbonates thereof, andremoving thus precipitated carbonates therefrom.

While the object of the present invention may also be accomplished bythe addition of compounds which produce C0,," in water, such as, e.g.,sodium carbonate or potassium carbonate, together with or in place ofC0,, in the following explanation of the present invention, specificreference will be made to the process in which the blowing of carbondioxide is adopted.

In general, when removing metals by precipitation as carbonates thereof,there is a possibility that the formation of precipitation of metalsoccurs only insufficiently in a low-pH region, due to the formation ofbicarbonates having higher solubilities than those of carbonates.

In order to prevent this from occurring, it is preferable in the processof this invention that the alkalinity of the catholyte be adjustedspecifically to have a pH value of not lower than 10, before, during, orafter blowing a gaseous carbon dioxide thereinto.

However, when a catholyte containing acrylonitrile is made alkaline,particularly at a pH higher than l0, acrylonitrile contained thereintends to be converted into biscyanoethyl ether or polymerized orhydrolyzed. Thus, it is preferable that a catholyte from whichacrylonitrile has been removed beforehand by distillation or the likeoperation be employed in the process of this invention.

Particularly preferable catholytes for use in the process of the presentinvention are ones obtained in the electrolysis of acrylonitrile andadiponitrile in a suspension state as disclosed in Dutch Pat.Specification No. 6,708,254, having been freed from not onlyacrylonitrile but also adiponitrile according to such means asquiescence, centrifuge, heat condensation, filteration-condensation,etc. Because, when such catholytes as referred to above are used, theundesirable hydrolysis of adiponitrile and byproducts formed byelectrolysis such as propionitrile and Z-cyanoethyl adiponitirle can beprevented, even if pH values thereof are increased to l0 or higher inthe process of this invention.

As described above, the process of this invention is preferablyapplicable to metal ion-containing catholytes free from acrylonitrile,

Now, the mode of working of a series of operations involving thepurification process of the present invention will be ex plained in thefollowing:

First, procedures for making a catholyte alkaline by electrolysisthereof will be explained as follows:

A metal ion-containing catholyte having been freed from acrylonitrile isintroduced to a cathode compartment of an electrolytic cell divided byan anion exchange membrane to form an anode compartment and the cathodecompartment. As hydrogen gas is generated from a cathode, pH value ofthe catholyte is elevated.

Although carbonates of metals may be formed by flowing a gaseous carbondioxide into the catholyte thus made alkaline and discharged from thecell, it is preferable that carbon dioxide gas be blown into thecatholyte entering the cell followed by making the same alkaline, sincethe concentration of alkali formed at the boundary surface of thecathode is quite high so that the rate of precipitation of carbonatesmay be greatly accelerated.

It is preferable to employ an anion exchange membrane as a diaphragm inan electrolytic cell, since by so doing hydrogen ions derived from ananode compartment and other ions are prevented from migrating into acathode compartment. Any anion exchange membranes conventionally usedmay be conveniently employed in the process of this invention. Forexample, membranes having quaternary ammonium groups obtained bychloromethylating a styrene-divinylbenzene copolymer matrix followed byquaternizing, or those obtained by copolymerizing styrene, vinylpyridine and divinylbenzene followed by quatemizing, inclusive ofuniform membranes and nonuniform membranes having an inserted corematerial, may be conveniently used.

Although anion exchange membranes are most preferable as a diaphragm asdescribed above, other diaphragms than anion exchange membranes may alsobe used. For example, it is possible to use a cation exchange membraneas a diaphragm, if a neutral or alkaline solution of a supportingelectrolyte is employed as an anolyte.

Any materials having a low hydrogen overvoltage and a resistance tocorrosion may be used for a cathode. Preferable materials for a cathodeinclude, for example, iron, stainless steel, nickel plate andnickel-plated iron plate.

In order to elevate an alkalinity of a metal ion-containing catholyte inthe process of the present invention, there may be adopted anothermethod in which the catholyte is passed through an anion exchangemembrane of hydroxy group type. In this method, the anion exchangemembrane after used may be regenerated by using sodium hydroxide or anaqueous ammonia. In this instance, however, the use of a supportingelectrolyte having an oleophilic anion such as, e.g., aryl sulfonate, isnot preferable, because it makes the regeneration of the anion exchangemembrane difficult.

There is still another method of making a catholyte alkaline in whichthe alkalinity is elevated by the incorporation of sodium hydroxide,slaked lime or the like thereinto. When quaternary ammonium sulfates areused as a supporting electrolyte, the incorporation of slaked lime isparticularly preferable since the alkalinity is elevated with theprecipitation of calcium sulfate.

in order to elevate the alkalinity of a catholyte in the process of thepresent invention, there may be adopted various methods other thandescribed above.

When applying the process of this invention to a catholyte containing asupporting electrolyte having sulfate ions, the alkalinity may beelevated to a pH value of higher than 10, after removing acrylonitriletherefrom. As for a catholyte in a suspension state, it is preferablethat first acrylonitrile be removed therefrom, second the catholyte bephase separated into an oil phase and an aqueous phase, third thealkalinity of thus separated aqueous phase alone be elevated to a pHvalue higher than according to the methods as described above and last agaseous carbon dioxide be blown thereinto.

Although a high alkalinity of the catholyte is desirable in the processof this invention since the higher the alkalinity becomes the more thecarbonates precipitate, a pH value of from about 11 to about 13 is mostpreferable from the economical point of view. Also, since the lower theconcentration of a supporting electrolyte in the catholyte to be treatedbecomes, the higher the rate of precipitation of the carbonate becomes,it is preferable that the concentration of the supporting electrolyte isnot higher than 25 percent.

In order to expedite the removal of iron ions from a catholyte, it issuggested that first the alkalinity of the catholyte be elevated andsubsequently an operation of exposing the catholyte to air therebyoxidizing iron ions being present therein and the operation of blowing agaseous carbon dioxide be combined together.

Carbonates and/or hydroxides thus formed according to the process of thepresent invention are then removed from a catholyte by sedimentation,filteration or centrifuge.

The catholyte thus treated in accordance with the present invention hasa pH value of higher than 10. However, a catholyte used in theproduction of adiponitrile by the electrolytic hydrodimerization ofacrylonitrile is required to have a pH value of lower than 10 in orderto prevent the byformation of biscyanoethyl ether. Thus, the treatedcatholyte is adjusted to have the desired pH value by adding an acidthereto.

When the presence of carbon dioxide ions in a catholyte is undesirable,carbon dioxide gas may be removed therefrom by blowing air thereintoafter lowering the pH to value of lower than 4.

As described above, in accordance with the present invention, theconcentration of metal ions in a catholyte obtained in the production ofadiponitrile by the electrolytic hydrodimerization of acrylonitrile canbe maintained at a low level, e.g. lower than about 2 ppm, withconsequential decrease in the precipitation of metals on the surface ofthe cathode. As a result, adiponitrile can be obtained in high yieldsfor a prolonged period of time. 4

The process of the present invention is applicable to the process asdescribed in the US. Pat. No. 3,l93,48l referred to above in which anaqueous solution containing acrylonitrile and adiponitrile in highconcentrations is electrolyzed to effect the electrolytichydrodimerization using a quaternary ammonium salt having oleophilicanions as a supporting electrolyte.

It also is applicable to the process as disclosed in the Dutch Pat.Specification No. 6,708,254 referred to above in which the electrolysisis carried out using a quaternary ammonium salt having nonoleophilicanions as a supporting electrolyte while maintaining acrylonitrile andadiponitrile in the form of a suspension.

in addition, the process of the present invention is widely applicableto other processes for producing adiponitrile from acrylonitrile by theelectrolysis thereof than referred to above.

In general, in practicing the process of the present invention, norestriction is imposed on the concentration of acrylonitrile andadiponitrile in the catholyte, type and concentration of the supportingsalt employed.

in accordance with the present invention, a catholyte need not be heatedto any particularly high temperatures and the purification of thecatholyte, i.e. removal of metals therefrom, can be accomplished atabout room temperature or at a temperature at which the electrolysis iseffected. Thus, the process of this invention is not accompanied by thedrawbacks of the prior art referred to hereinbefore.

Another advantage of the process of this invention is that since, ingeneral, metal carbonates have smaller solubilities in water than thoseof metal hydroxides, the removal of precipitates of metal carbonates canbe achieved easier than those of metal hydroxides.

The working of the process of this invention will be explained byreferring to the accompanying drawing which illustrates a typical flowsheet of the present process. It should not be construed, however, thatthe present invention is restricted by the following description.

Numeral 2 designates an electrolytic cell divided by a cation exchangemembrane to form a cathode compartment and an anode compartment employedfor the production of adiponitrile by electrolytically hydrodimerizingacrylonitrile. An anolyte in the cell 2 is recycled between the anodecompartment of the cell 2 and an anolyte tank 1. Between the cathodecompartment of the cell 2 and a catholyte tank 3 is recycled acatholyte.

The catholyte consists of an aqueous phase mainly comprising an aqueoussolution of a quaternary ammonium salt and an oil phase mainlycomprising acrylonitrile and adiponitrile suspended therein.

A part of the catholyte of the catholyte tank 3 is supplied to anacrylonitrile stripper 4 where acrylonitrile is distilled and thusdistilled acrylonitrile is recycled to the catholyte tank 3. Suspension,i.e. catholyte, from which acrylonitrile has been removed is separatedinto two phases in quiescence in a decanter 5, and an oil phase mainlycomprising adiponitrile is fed to a purification process through line 6.An aqueous phase is supplied to a tank 7.

Numeral l0 designates an electrolytic cell divided by an anion exchangemembrane to form an anode compartment and a cathode compartment providedfor increasing an alkalinity of the catholyte fed from the tank 7.Between the anode compartment of the cell and an anolyte tank 11 isrecycled an anolyte. A catholyte in the cell 10 is supplied from thetank 7 and is recycled between the cathode compartment of the cell 10and the tank 7. Into the tank 7 is blown carbon dioxide gas from a line8 and hydrogen gas generated in the cell 10 is discharged from a line 9.

A part of liquid in the tank 7 is fed to a packed tower 12 where theliquid is brought into contact with air 13 in a countercurrent fashionto oxidize iron ions present in the catholyte.

Subsequently, carbonates and/or hydroxides of metals formed are filteredoff in a filter l4 and the catholyte freed from contaminants is storedin a tank 15 where the pH thereof is adjusted at about 4 by the additionof an acid from a line 16. Then, the adjusted catholyte is supplied to apacked tower 18 where the same is countercurrently contacted with airsupplied from a line 17 thereby removing carbon dioxide therefrom. Theresulting purified catholyte thus obtained is recycled to the catholytetank 3.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The following examples willserve to illustrate the present invention more fully.

EXAMPLE 1 Referring to the fiowsheet of the accompanying drawing, theanolyte tank 1 was filled with a 2N sulfuric acid and the catholyte tank3 was filled with an emulsion having the following composition:

Oil phase by weight) Acrylonitrile 20 Adiponitrile 70 Electrolysisbyproducts 5 Water 5 Aqueous phase Tetraethylammonlum sulfate l0Acrylonitrile. adiponitrile and electrolysis byproducts 8 Water BalancepH 4 Volume ratio ofoil phase to aqueous phase 2:8

A part of the emulsion was supplied to the acrylonitrile stripper 4 toremove acrylonitrile therefrom and the bottom was separated inquiescence in the decanter 5. At this point, the concentration ofacrylonitrile remaining in the aqueous phase was lower than 0.05percent.

The liquid, i.e. the aqueous phase free from acrylonitrile, is suppliedto the electrolysis cell 10 as a catholyte. The electrolytic cell 10included a cathode made of stainless steel, an anode made of lead alloyand a diaphragm of an anion exchange membrane. A 2N sulfuric acid wasused as an anolyte. In the electrolytic cell, the electrolysis wasconducted with an electric current of IO ampere/dm. and alkali wasformed with an electric current efficiency of about 50 percent. Then,carbon dioxide gas was blown into the catholyte from the line 8 whilemaintaining the pH of the catholyte in the tank 7 at a value of 12.

A part of the resulting catholyte thus obtained was fed to the packedtower 12 where the same is subjected to aeration. Subsequently,impurities were removed from the catholyte in the filter 14 and the pHof the catholyte is adjusted at a value of 4 in the tank 15 by addingsulfuric acid thereto. By conducting the aeration in the packed towerl8, residual carbon dioxide gas is removed therefrom and the resultingpurified catholyte is recycled to the catholyte tank 3.

The following effects were brought about by the treatment describedabove.

In the flowsheet referred to above, when no electrolytic cell 10 wasemployed and no blowing of carbon dioxide was conducted, theconcentrations of calcium ions, magnesium ions, lead ions and iron ionsin the aqueous phase of the catholyte in the catholyte tank 3 were 15p.p.m., 3 p.p.m., 5 p.p.m. and 3 p.p.m., respectively.

Electrolytic hydrodimerization of acrylonitrile was conducted in theelectrolytic cell 2 under such conditions as described above for about400 hours with a result that the conversion of acrylonitrile toadiponitrile was lowered to about percent.

In contradistinction, when the electrolytic cell 10 and the blowing ofcarbon dioxide were employed thereby reducing the concentration of totalmetal ions such as calcium, magnesium, lead and iron, in the catholytetank 3 to lower than about 2 p.p.m., the conversion of acrylonitrile toadiponitrile was maintained as high as 92 percent even after thecontinuous operation for about 400 hours.

In the instant example, the same effect as described above was obtainedby using a cation exchange membrane as a diaphragm in the electrolyticcell and tetraethylammonium chloride as an anolyte.

EXAMPLE 2 Example 1 was repeated according to the same procedures asdescribed therein except that an anion exchange tower was providedinstead of the electrolytic cell 10 and the anolyte tank 11. An anionexchange resin having been converted to hydroxy group type by sodiumhydroxide was used after a thorough washing with water. The liquid fromthe tank 7 was passed through a layer of the resin with blowing ofcarbon dioxide gas thereinto thereby maintaining the pH of the liquid ata value of 12.

The same effect as in example I was obtained.

EXAMPLE 3 In the operation of the example l, the pH of the catholyte wasadjusted at about 13 by adding slaked lime to the aqueous phase of thedecanter 5 in an amount of 5 kg. per m. of the aqueous phase, instead ofemploying the electrolytic cell 10, anolyte tank 11 and tank 7.

After filtering off calcium sulfate which had precipitated, carbondioxide gas was blown into the liquid to make the pH at a value of 12.The resulting liquid was supplied to the packed tower 12.

By the treatment described above, the same effect as in example l wasobtained. In this example, the same effect was obtained by adding sodiumcarbonate to the liquid instead of blowing carbon dioxide thereinto.

We claim:

l. A process for the continuous production of adiponitrile by subjectingto electrolysis a catholyte comprising acrylonitrile, an electrolyte andwater and being an emulsion of an oil phase and an aqueous phase,wherein the catholyte is cycled through an electrolytic cell and areservoir vessel, part of the catholyte is continuously orintermittently removed from the reservoir vessel, acrylonitrile and saidoil phase containing adiponitn'le are removed from said catholyte,subsequently treated with carbon dioxide under alkaline condition at apH not lower than 10 whereupon contaminant metal ions contained thereinare precipitated in the form of the corresponding separating theprecipitated carbonate and recycling the resulting purified solution ofthe supporting electrolyte as a catholyte, and the separatedacrylonitrile is also returned to the reservoir.

2. Process according to claim 1 wherein said aqueous phase of thecatholyte contains a quaternary ammonium salt having nonoleophilicanions as a supporting electrolyte.

3. Process according to claim 1, wherein said alkali is selected fromsodium hydroxide and slaked lime.

4. Process according to claim 1, wherein said aqueous phase of thecatholyte treated with carbon dioxide has a pH ranging from I] to 13 andthe concentration of the electrolyte contained in said aqueous phase isnot higher than 25 percent.

5. Process according to claim 1, wherein acrylonitrile and said oilphase are separated from said aqueous phase of said catholyte and saidaqueous phase is electrolyzed as a catholyte in an electrolyzer havingan anode compartment and a cathode compartment separated from each otherwith a diaphragm whereupon the pH of said aqueous phase is increased tonot lower than 10, prior to, simultaneously with, or after the treatmentof said aqueous phase with carbon dioxide therein.

6. Process according to claim I, wherein acrylonitrile and said oilphase are separated from said aqueous phase of said catholyte and saidaqueous phase is passed through a hydroxy group type anion exchangeresin whereupon the pH of said aqueous phase is increased to not lowerthan 10, prior to, simultaneously with, or after the treatment of saidaqueous phase with carbon dioxide.

7. Process according to claim 1, wherein an alkali is added to saidaqueous phase of the catholyte in sufi'icient quantity to raise the pHthereof to not lower than 10, prior to, simultaneously with, or afterthe treatment of said aqueous phase with carbon dioxide.

Dated October 26 197].

Pat ent No. 3 6 322 Inventor(s) NAOMI SEKO et al It is certified thaterror appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

Column 7, lines 12-15 "separating...reservoir should be deleted andreplaced with ---metal carbonate,

metal carbonate so precipitated is returned to the reservoir for reuse,and the separated Signed and sealed this 9th day of April 197A.

(SEAL) Attest:

C. MARSHALL DANN EDWARD I LFLETGHERJR.

Commissioner of Patents Attesting Officer DRM PO-1D50 (10-69) 9 us.GOVERNMENT PRINTING omc: nu o-sn-su.

2. Process according to claim 1, wherein said aqueous phase of thecatholyte contains a quaternary ammonium salt having nonoleophilicanions as a supporting electrolyte.
 3. Process according to claim 1,wherein said alkali is selected from sodium hydroxide and slaked lime.4. Process according to claim 1, wherein said aqueous phase of thecatholyte treated with carbon dioxide has a pH ranging from 11 to 13 andthe concentration of the electrolyte contained in said aqueous phase isnot higher than 25 percent.
 5. Process according to claim 1, whereinacrylonitrile and said oil phase are separated from said aqueous phaseof said catholyte and said aqueous phase is electrolyzed as a catholytein an electrolyzer having an anode compartment and a cathode compartmentseparated from each other with a diaphragm whereupon the pH of saidaqueous phase is increased to not lower than 10, prior to,simultaneously with, or after the treatment of said aqueous phase withcarbon dioxide therein.
 6. Process according to claim 1, whereinacrylonitrile and said oil phase are separated from said aqueous phaseof said catholyte and said aqueous phase is passed through a hydroxygroup type anion exchange resin whereupon the pH of said aqueous phaseis increased to not lower than 10, prior to, simultaneously with, orafter the treatment of said aqueous phase with carbon dioxide. 7.Process according to claim 1, wherein an alkali is added to said aqueousphase of the catholyte in sufficient quantity to raise the pH thereof tonot lower than 10, prior to, simultaneously with, or after the treatmentof said aqueous phase with carbon dioxide.